Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates generally to sheet handling
apparatus and more particularly concerns a decurling device for taking out
the curl imparted to the sheet material incident to previous operations.
In the manufacture of paper, due to the normal fiber distribution
pattern and orientation, there exists an inherent tendency to cause the paper
to curl. In various sheet handling apparatus, this tendency is aggravated
since the sheet material is caused to wrap around curved surfaces which,
in combination with other operating parameters, causes the paper sheet
material to curl. This is particularly true in the operation of printing
10 presses, for example of the offset type, where the sheet material, though
carried on the impression cylinder, tends to adhere to the moistened surface
of the blanket cylinder. Since the leading edge of the sheet is cl-amped in the
impression cylinder there is little tendency for the first part of the sheet to
curl. However, as the sheet passes progressively through the printing couple,
the tendency to curl increases. For this reason, and as those experienced
in the printing art will recognize, this curling tendency is referred to as
"tail-end hook". The amount of curl or tail-end hook is further influenced
by the moisture content of the paper and the relative humidity of the
surrounding atmosphere, the weight and type of the sheet material and the
20 viscosity and coverage of the ink. Lighter weight paper tends to curl more
and the tendency of the paper to curl increases with the viscosity of the ink
as well as the area covered by the ink.
It will also be recognized by those skilled in the art that curled
sheets create many subsequent handling problems. Thus it is difficult to
get successive sheets to stack properly even if jogging equipment is
employed. Improperly stacked sheets, of course, cause many additional
problems in folding, cutting and binding equipment as well as registration
difficulties if subsequent impressions are to be printed on the sheet material.
--2-- ~
s
In view of the foregoing, various attempts have been made in the
past to develop apparatus to take the curl out of sheet material. One
approach is shown in U. S. Patent No. 3, 6~1, 703 issued May 9, 1972 which
employs a series of rolls about which a web is trained. Another approach
is shown in U.S. Patent No. 3,076,492 issued February 5, 1963 which employs
a vacuum trough having a continuous slot therein. In both of these patents
the material being processed is given a reverse bend which, if properly
regulated, removes the effect of the curl previously imparted. While the
latter patent with its relatively shallow trough and open vacuum slot may
be particularly suited for decurling paper sheet material laminated to a
metal foil, it has not proven entirely satisfactory for decurling the wide
range of unbacked paper stock normally used in the printing arts. Moreover,
the metallic surfaces of such devices may become -flawed or burred and leave
scratches on the face oE the subsequent sheets of material that are drawn
across it. This is particularly objectionable when such a burr scratches the
face of a previously printed sheet. Another objection to devices such as
disclosed in the latter-mentioned patent is that as the ~trailing edge of the sheet
material is drawn across the slot a verg audible popping noise is generated
and at high production speeds this is very irritating to the surrounding press
room perso~mel. Also when light weight sheet is sucked into such a slot it
tends to feather the end of the sheet.
By the present invention an improved device for decurling curled
sheets in a sheet handling apparatus having means for positively moving
successive sheets along a delivery path is provided including an elongated
suction bar disposed across the delivery path substantially transverse to the
direction of sheet ~lovement, characterized in that the suction bar
has a substantiall~7 M-shaped cross section with two smoothly
rounded crests whose curvature is in the same direction as the
~ ,, ':.
-3-
~5~ 5
~.
curl of the sheets and a rounded U-shaped trough between
the crests having a multiplicity of apertures therein, with
means bridging the legs of the M-shaped cross section below
the trough for forming a vacuum manifold closed except for
the apertures and means for drawing a vacuum in the manifold.
The radius of the trough is substantially less than the
radius of the crests so that as a curled sheet is moved
across the crests and drawn into the trough by the vacuum
it is progressively bent in a compound reverse curvature the
sharpest bend of which is opposite the original direction
of curl. The crests and trough are coated with a smooth
hard plastic gel coat which contains a dlspersion of fine
carbon particles and the bar has di-electric properties to
electrically destaticize the sheets.
In a specific embodiment of the invention the ratio
of the radii of the crests to the radius of the trough is
grea-ter than about 5 to 1 and the depth of the trough is
substantially equal to one-half the span between the crests
along a line of common tangency thereto. Also according to
the invention, the aper-tures are slightly countersunk in the
bottom of the trough and the suction bar is preferably ma~e
of a reinforced plastic material and the crests, trough and
apertures are provided with a hard, smooth gel coat containing
a dispersion of fine carbon particles and the bar has di-
electric properties to electrically destaticize the sheets.
Figure 1 is a schematic side elevation of a sheet
handling apparatus incorporating the present invention in
association with a printing press;
Fig. 2 is an enlarged fragmentary schematic view
of the impression and blanket cylinders of the printing press
with a sheet of material, in exaggeratecl form, passing there-
throuyh;
sb/J~
~L~)5~'~35
Fig. 3 is a plan view of the decurling device
of the present invention;
Fig. 4 is an illustration par-tly in section
and partly in elevation
A - 4a -
sb/,-~
as seen substantially along the lines 4-4 in Fig. 3; and,
Fig. 5 is an enlarged section as seen substantially along line
5-5 in Fig. 4 and also including a schematic diagram of the vacuum source.
While the invention will be disclosed in connection with certain
preferred embodiments, it will be understood that I do not intend to limit
the invention to those embodiments. On the contraryJ I intend to cover all
alternatives, modifications and equivalents as may be included within the
spirit and scope of the invention as defined by the appended claims.
Turning now to the drawings, there is shown inFig. 1, in highly
schematic form, a printing press 10 and a sheet feeding and delivery
apparatus 20 with which the present invention is associated. In the
illustrated embodiment, the printing press 10 is of the offset lithographic
type and includes a plate cylinder 12J a blanket cylinder 14 and an
impression cylinder 16. It will also be understood that the printing press
10 includes an ink supply and a dampening solution supply together with
respective ink and dampening feed and form rollers (not shown) for
transferring the ink and dampening solution to the plate cylinder 12 from
their respective sources in a regulat ed and properly distributed manner.
The ink impression on the lithographic plate mounted on the plate cylinder
12 is transferred to a rubber blanket 18 which forms the surface of the
blanket roll 14 and this image is then transferred to the surface of a sheet
S of paper or the like drawn through the nip between the blanket cylinder
14 and the impression cylinder 16.
In schematic form, the sheet feeding and delivery apparatus 20
includes a pair of endless transport chains 22 disposed in parallel relation
on either side of the press 10 (only one of which is shown) and trained in
orbital fashion around a drive sprocket 24 and a driven sprocket 26.
[ounted between the parallel chains 22 are a plurality of gripper bars 28
Z3~
which receive the individual sheets S from a source (not shown) and deliver
them to the impression cylinder 16 where they are released from the
gripper bar 28 and fed into a clamping mechanism 30 on the impression
cylinder 16. The sheet S is then carried by the impression cylinder 16
through the nip formed with the blanket cylinder 14 as the chains 22 continue
to move to the let as seen in Fig. 1. The chains 22 are timed such that
a subsequent gripper 28 passes adjacent the sheet clamping mechanism 30
on the impression cylinder as the cylinder carries the sheet S downwardly
on the left-hand side of the press as seen in Fig. 1. The sheet S is then
released by the clamping mechanism 30 and engaged by the subsequent
gripper bar 28 which carries it through the initial delivery section extending
to the left-hand side of the press 10.
As is co~rlmon in many sheet delivery arrangements associated
with printing presses, and as shown schematically in Fig. 1, the final
delivery section is vertically offset from the initial delivery section.
Accordingly, pairs of direction-reversing idle sprockets 32 are provided
in both the forward and return reaches of the chains 22. To support the
sheets S between the parallel chains 22 one or more plates or rails 34 are
provided in the sheet feeding section. As the sheet S passes around the
driven sprocket 26 in the delivery section the gripper bar 28 is opened and
the sheet is discharged to a stacking mechanism (now shown) which deposits
the sheet on the top of a stack of previously delivered sheets.
Referring now to Fig. 2, there is shown in exaggerated form a
sheet S as it passes through the nip between the blanket cylinder 14 and the
impression cylinder 16. Because of the ink on the rubber surface 18 of the
blanket cylinder, the sheet S tends to adhere to the rubber blanket 18 after
it leaves the nip of the rolls 14, 16. The sheet is then drawn downwardly
by the clamping mechanism 30 on the impression roll 16 and this creates a
bend b in the sheet S as it is pulled away from the blanket 18. It will be
understood that the radius of this bend b depends on a number of things
such as: the relative humidity of the paper; the pressure at the printing
nip; the thickness of the paper; thinner paper being more flexible and thus
taking a sharper bend; the viscosi-ty of the ink; and, the relative area of
the sheet covered by the ink, together with the inherent characteristics
of the paper~ The result of this bend b is to impart to the sheet S a curl
which is progressively more pronounced toward the trailing edge of the
sheet and is known in the art as "tail-end hook".
One such curled sheet S is shown, somewhat exaggerated, just
to the left of center in Fig. 1. As those skilled in the art will appreciate,
such curled sheets are difficult to stack and handle in subsequent sheet
feeding operations. Non-aligned stacks of sheets also cause further
problems in folding, cutting and binding equipment and cause registration
difficulties if the curled sheets are to be printed with additional impressions.
In accordance with the present invention, an improved decurler
device 40 is provided for the sheet handling apparatus 20 to take the curl
out of the sheets S. As shown schematically in Fig~ 1, the decurler device
40 is located below the forward reach of the chains 22 in -the final delivery
section following the upper direction-reversing sprockets 32. The gripper
bars 28 are pivo-tally mounted between the side chains 22 and they pass
above the decurler device 40 but the sheet material S swings down and is
drawn across the top of the decurler device.
Referring also-to Figs. 3-5, the decurling device 40 includes an
elongated suction bar 42 disposed across the delivery path substantiallg
transverse to the direction of sheet movement. In the preferred
embodiment and as shown in Fig. 5, the suction bar 42 has a substantially
M-shaped cross section with two smoothly rounded crests 4'L separated by
~)S~'~3~
a rounded U-shaped trough 46. A bottom panel 48 bridges the legs of the
M-shaped bar 42 and the ends are closed by plates 50.
To draw the sheet S into the trough 46 of the bar 42 a vacuum
pump 52 is coupled to the bar 42 through a conduit 54 and an end fitting 56
threaded into the bottom panel 48. As a vacuum is drawn by the pump 52,
air i9 sucked into the bar 42 through a multiplicity of apertures 58 formed
in the bottom of the trough 46. As the sheet S is pulled across the crests
44 by the gripper bar 28 the suction in the bar 42 pulls the sheet S into the
trough 46 giving the sheet a reverse bend with respect to its original curl.
1 0 In this connection, it will be noted that the curvature o`f the crests 44 is in
the same direction as the curl of the sheets S and the radius of the rounded
trough 46 is substantially less than the radius of the crests 44 so that as the
sheet is drawn into the trough 46 by the vacuum it is progressively bent in a
compound reverse curvature with the sharpest bend being opposite the
original direction of curl.
A preferred form of the vacuum bar 42 has dimensions
substantially as shown in Figs. 3 and 4, i. e., an overall height and width of
approximately 1 and 1/2 inches and with the apertures 58 being 5/64 inch in
diameter and spaced on 1/4 inch centers. While the apertures 58 extend
20 substantially across the trough 46 as viewed in the direction of sheet movement,
they occupy less than one-third of the length of the trough and the solid
material in the bottom of the trough 46 supports the sheet S even when it is
drawn fully into the trough. It has been found that this substantially eliminates
the "popping" noise that is created when the trailing edge of a sheet is
drawn across a continuous slot such as shown in F`ig. 2 of the above-
mentioned Patent No. 3, 076, 492.
While Fig. 5 is a somewhat enlarged view, it will be seen that
the depth of the trough 46 is substantially one-half the distance between the
--8--
3~
crests 44 along a line tangent to both crests. ~oreover, the radii of the
crests 44 are large relative to the radius of the trough 46 and in the preferred
embodiment this ratio is greater than about 5 to 1. It will also be appreciated
that the large interior cross section of the bar 42 serves as an equalizing
manifold to distribute the vacuum substantially informly along the length of
the bar so that the suction at each of the apertures 58 is about the same.
With a vacuum of about 20 inches Hg. drawn by the pump 52 the
decurling device 40 of the present invention has been found to be effective
in taking the curl out of paper sheet material varying in weight from about
10 40 Ib. to 100 Ib. per ream and over a wide range of press operating
conditions. However, to make the decurling device 40 even more versatile
it is also desirable to control the vacuum level. In the illustrated
embodiment and as schematically shown in Fig. 5, this is accomplished by
driving the vacuum pump 52 with a variable speed electric motor 60 under
control of a regulator 62.
As previously mentioned, lighter weight sheet material tends to
take on more curl than the heavier weight material--other conditions being
equal. To take the curl out of such light weight material it is necessary to
give it a relatively sharp bend as it crosses the suction bar 42 and thus it is
20 drawn substantially completely into the trough 46. In contrast, if heavy
weight sheets are given such a sharp bend the likelihood increases that it will
"break the back" of the sheets or fracture the coating, if coated sheets are
employed. Of course, the heavier sheet material bends less than light
material and it has been found that a decurler device 40 having the parameters
mentioned above provides a good balance for a wide range of sheet material
and operating conditions.
Pursuant to another aspect of the present invention the vacuum
bar 42 is preferably made o~E fiber glass reinforced plastic material provided
23S
with a smooth, hard wear resistant gel coat. In its preferred form, the bar
42 has a tensile strength of about 45, 000 psi. with elongation of only about
2% and the gel coat has a Barcol hardness of about 43. The apertures 58 are
preferably slightly countersunk at the bottom of the trough 46 and then given
an additional light coat of the plastic gel coat material. The outer surface
of the bar 42 may then be lightly polished with #600 grit sandpaper and water to
leave an-extremely smooth and "glossy" surface. The inside surface of the
bar is also polished and waxed to prevent the accumulation of dust and paper
particles from clogging the suction channel. In use it has been found that
10 the surface of such bars 42 is substantially easier to initially prepare and
maintain free from flaws than metal bars. It will be appreciated of course
that even with expensive and sophisticated tooling and machining techniques
it is extremely difficult to form metal parts having compound curvatures
without lea~ing score lines, small burrs and other surface defects. Moreover,
paper is abrasive and as the metal surface is worn other flaws and defects
may be exposed. If a flaw or burr is exposed it may scratch the surface of
sheets drawn across it. This is particularly objectionable if the sheet surface
has been previously printed. In addition, such burrs, defects and score lines
tend to trap and collect dust and other particles, and if these are abrasive,
20 further scratching of the sheets occur. Not only is the hard plastic surface
of the preferred bar 42 less subject to such flaws and defects, but it may also
be more quickly and conveniently retouched when this is required. Thus, it
may simply be sanded down with #600 grit sandpaper and water and then
recoated with additional plastic gel coat material.
Finally, as a further feature of the present invention the gel
coat (illustrated as a layer in Fig. 5) may contain a dispersion of fine carbon
or graphite particles. The resulting di-electric properties of a bar 42 with
such a coating has been found to be effective in eliminating the electrostatic
-10-
1~5~Z;~S
charge that builds up in the paper sheet material as a result of prior
handling operations. Thus, the bar 42 is not only capable of delivering Elat,
decurled sheets, but also, sheets that are essentially static free. Both of
these factors, of,course, contribute in making subsequent sheet handling
operations much easier.
-11 -
